Top 15 JavaScript Interview Questions and Answers (With Code Examples)

JavaScript is the backbone of web development, and whether you're applying for a frontend or full-stack role, you will face JavaScript questions in your interview. In this post, we’ve compiled the top 15 most commonly asked JavaScript interview questions — from the basics to tricky advanced topics, complete with clear answers and examples.

📌 Table of Contents

1. What is the difference between == and ===?
2. What are closures in JavaScript?
3. What is hoisting?
4. What are callback functions?
5. What is the event loop?
6. Difference between var, let, and const
7. What is a Promise?
8. What is async/await?
9. What is the difference between null and undefined?
10. Explain “this” keyword in JavaScript
11. What is prototypal inheritance?
12. What are arrow functions?
13. Explain setTimeout and setInterval
14. What is a higher-order function?
15. How does JavaScript handle asynchronous code?

---

❓ Q1: What is the difference between == and ===?

✅ == compares values after type coercion, === compares both value and type.

5 == '5' // true
5 === '5' // false

❓ Q2: What are closures in JavaScript?

✅ A closure is a function that retains access to its lexical scope even after the outer function has returned.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  }
}
const counter = outer();
console.log(counter()); // 1

❓ Q3: What is hoisting?

✅ Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.

console.log(a); // undefined
var a = 10;

❓ Q4: What are callback functions?

✅ A callback is a function passed into another function as an argument.

function greet(name, callback) {
  console.log("Hi " + name);
  callback();
}
greet("Ankur", () => console.log("Welcome!"));

❓ Q5: What is the event loop?

✅ The event loop allows JavaScript to perform non-blocking operations by offloading operations to the browser APIs and handling them asynchronously.

❓ Q6: Difference between var, let, and const

✅ var is function-scoped, let and const are block-scoped. const cannot be reassigned.

❓ Q7: What is a Promise?

✅ A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation.

let promise = new Promise((resolve, reject) => {
  resolve("Done!");
});
promise.then(result => console.log(result));

❓ Q8: What is async/await?

✅ async and await make asynchronous code look synchronous.

async function fetchData() {
  let res = await fetch("api/data");
  let data = await res.json();
}

❓ Q9: What is the difference between null and undefined?

✅ null means an intentional absence of any object value; undefined means a variable has been declared but not assigned.

❓ Q10: Explain “this” keyword in JavaScript

✅ this refers to the object that is executing the current function.

❓ Q11: What is prototypal inheritance?

✅ Objects can inherit properties and methods from another object via the prototype chain.

❓ Q12: What are arrow functions?

✅ Arrow functions are a shorter syntax for writing functions and don’t have their own this binding.

const add = (a, b) => a + b;

❓ Q13: Explain setTimeout and setInterval

✅ setTimeout() runs a function once after a delay, setInterval() runs it repeatedly at a given interval.

❓ Q14: What is a higher-order function?

✅ A function that takes another function as an argument or returns one.

function multiplier(factor) {
  return function(number) {
    return number * factor;
  };
}

❓ Q15: How does JavaScript handle asynchronous code?

✅ Through the event loop, using callbacks, promises, and async/await.

---

📣 Final Words

JavaScript is one of the most asked topics in web development interviews. Make sure you understand these concepts well and practice implementing them. This post gives you a solid base to build on.

Stay tuned: Next up – React Hooks Interview Questions and Answers!

📧 Got a topic request? Email me at interviewwithcode@gmail.com

---

📥 Free Resource (Coming Soon)

Download our PDF “50 JavaScript Questions with Answers” — Available for free subscribers!